The use of gyrators to simulate inductance in integrated circuits is well known. One application of gyrator circuits is in a line interface circuit between a telephone system user station and a telephone line (often called a loop circuit or a two wire circuit) extending to a local telephone system switch, such as a private automatic branch exchange (PABX). The gyrator develops the necessary complex impedance (i.e., inductance, capacitance, resistance) to match the two wire circuit impedance and to respond to application and removal of current from the PABX office battery to the two wire circuit to actuate a binary state indicator such as an opto-isolator. The indicator provides a two wire circuit status signal to the user's station equipment. That equipment may take different forms, and one is, e.g., a computer adapted in a known manner as a voice messaging system (VMS) (i.e., for operation in a telephone message service, or a so-called "voice mail" service). In such service, the user's equipment operates like a highly sophisticated answering machine. Additional background information on voice messaging systems is available in many publications, e.g., U.S. Pat. No. 4,926,462.
In some gyrator applications there is such a low impedance coupling between the gyrator and the opto-isolator that the coupling circuit loads the gyrator and distorts its impedance matching function. In addition, the signaling function is subject to erroneous operation due to effects of aging and temperature changes on the circuit components involved. The result is false operation of the opto-isolator when, e.g., either the two wire circuit current is too low to indicate a true two wire circuit connection or current ring out occurs when the PABX releases a two wire circuit at the end of a call. A false signal ("Falsing") due to ring out is especially troublesome because it disrupts a normal "wink off" function for disconnect in a way which forces an overall longer PABX/VMS system time out procedure to be used for disconnect. When the longer procedure is used, the PABX and VMS facilities are tied up for an undesirably long time which can result in blocking if it occurs on very many of the lines served. PABX systems seldom have any additional disconnect capability once a wink off function has been initiated. If user station equipment does not recognize the wink off and opens a relay contact to appear to be "on hook" to the PABX, the line remains in a "busy" state to the PABX and unusable until some other type of intervention takes place, i.e., human or system reset.